A Theoretical Study on Cool Flame Oxidation as an Effective Way for Fuel Reforming: Emphasis on Ignition Characteristics and Chemical Analysis

Cool flame oxidation of hydrocarbon fuels produces substantial intermediates with various levels of chemical reactivity, which can be used for combustion control of advanced engines with low-temperature combustion. Previous investigation has evaluated the performance of low-temperature reforming under high fuel-rich conditions, whereas the potential of a broader scope of reforming conditions (especially fuel-lean scenarios) is not clear. In this work, cool flame oxidation as a reforming way was proposed to investigate the effect of reforming products from various reforming conditions on ignition characteristics of typical operating conditions, and the role of the reforming products from typical reforming conditions on ignition characteristics of different operating conditions was clarified under engine-relevant conditions. The results show that the reforming products via cool flame oxidation are more sensitive to reforming temperature than pressure and equivalence ratio. Residence time also plays an important role in reforming products, and depending on the oxidation degree, two kinds of reforming products can be identified, i.e. mild reforming and severe reforming, manifesting distinct chemical reactivity. Meanwhile, it is found that different from the previous findings of very fuel-rich reforming conditions, the ignition delay time of different blends is significantly shortened under most operating conditions, except for some scenarios (e.g. the negative temperature coefficient region) manifesting a “cross-point” behavior. Moreover, the promotion on ignition processes becomes enhanced under higher temperature, higher pressure, leaner mixture, and severe reforming conditions. Globally, cool flame oxidation as a reforming way can reduce the ignition delay time by 30 ~ 50% under engine-relevant conditions. The current work can provide new perspectives of cool flame oxidation as an effective way to control advanced engine combustion processes.

烃类燃料的冷焰氧化（Cool Flame Oxidation）会生成大量具有不同化学活性水平的中间产物，这些产物可用于具备低温燃烧（Low-Temperature Combustion）技术的先进发动机的燃烧控制。既往研究已对高富燃料工况下的低温重整性能进行了评估，但更广泛范围的重整工况（尤其是贫燃料场景）的应用潜力尚不明确。本研究提出以冷焰氧化作为重整手段，探究不同重整工况下生成的重整产物对典型运行工况点火特性的影响，并在发动机相关工况下阐明了典型重整工况的产物对不同运行工况点火特性的作用机制。结果表明，通过冷焰氧化生成的重整产物对重整温度的敏感性高于对压力和当量比的敏感性。停留时间对重整产物的组成也具有重要影响，根据氧化程度的不同，可将重整产物分为轻度重整和重度重整两类，二者表现出截然不同的化学活性。与此同时，与既往针对高富燃料重整工况的研究结果不同，在大多数运行工况下，不同混合燃料的点火延迟时间均显著缩短，仅部分场景（如负温度系数（Negative Temperature Coefficient）区域）表现出“交叉点”特性。此外，在更高温度、更高压力、更稀薄的混合气以及重度重整工况下，对点火过程的促进作用会进一步增强。总体而言，在发动机相关工况下，以冷焰氧化作为重整手段可将点火延迟时间缩短30%~50%。本研究可为将冷焰氧化作为控制先进发动机燃烧过程的有效手段提供新的研究视角。